Fermentation is a metabolic process converting sugar to acids, gases and/or alcohol. It occurs in yeast and bacteria. In the strictest sense, fermentation is a form of anaerobic digestion. Fermentation occurs in the absence of oxygen (and the electron transport chain). Fermentation takes a reduced carbon source such as glucose to make products like lactic acid, acetate or ethanol.

Fermentation can more broadly refer to the bulk (larger-scale) growth of microorganisms on a growth medium. The science of fermentation is known as zymology. In the lab, fermentation as a form of bulk growth, involves the use of industrial-grade fermentors and media supplements and components to assist with optimized pH, oxygen levels, etc for a more controlled (and larger-scale) growth of microbial cultures. Agents added to the media include: vitamins, amino acids, detergents, sugars, and anti-foaming agents to optimize growth conditions.

Fermentation has been used by humans for the production of food and beverages since the Neolithic period. Fermentation is employed for preservation in a process that produces lactic acid as found in sour foods such as pickled cucumbers, kimchi and yogurt. Fermentation is used for producing alcoholic beverages such as wine and beer.